include Stdlib.List

let flat_map f xs =
  let rec iter = function
    | [] -> []
    | x :: xs -> append_then_iter xs (f x)
  [@@tail_mod_cons]
  and append_then_iter xs = function
    | [] -> iter xs
    | y :: ys -> y :: append_then_iter xs ys
  [@@tail_mod_cons]
  in
  iter xs

let iter_up_to f xs ~limit =
  let rec iter n = function
    | x :: xs when n > 0 ->
      f x; iter (n - 1) xs
    | _ -> ()
  in
  iter limit xs